Reducing nitrogen surplus from dairy farms. Effects of feeding and management.

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Abstract:

The objective of the present paper is to review the factors which can affect N flow and surplus both at farm and at cow
level in order to point out areas with scope for future improvement. Special attention is given to management factors and
feeding. Besides information from the literature the paper is based on meta-analyses of our own and published results. With
regard to effects of production systems, mainly Danish surveys have been chosen as examples demonstrating the effects
obtained under practical conditions. A positive correlation between stocking rate and N surplus per hectare at farm gate level
is demonstrated, but there is also a considerable variation in N surplus per hectare at a given stocking rate. A number of
factors influencing N surplus and loss have been identified, and their impact on N surplus and production efficiency has been
estimated. N excretion per animal is an important factor for N turnover at farm level. Analysis of herd data indicates that
feeding strategy, breed and milk yield, together with energy conversion and the protein content of the diet, are important
factors explaining N excretion and N efficiency of cows. Reduction of N intake by optimal synchronisation of energy and
protein supply over time, especially in pasture-based systems, is one way of reducing N excretion from cows. Furthermore,
the ideal profile of absorbed amino acids should be identified, and models to estimate amino acid supply to the intestine
should be further improved. The effect of reducing N excretion from cows has to be evaluated at farm level as manure is
used as fertiliser for crop production. Overall, it seems possible to reduce the N surplus through better management and
feeding without reducing production efficiency.